1. Field of the Invention
The present invention relates to the equipment and methods in oil field operations. Particularly, the invention relates to pumps.
2. Background of the Related Art
Helical gear pumps, typically known as progressive cavity pumps/motors (herein PCPs), are frequently used in oil field applications, for pumping fluids or driving downhole equipment in the wellbore. A typical PCP is designed according to the basics of a gear mechanism patented by Moineau in U.S. Pat. No. 1,892,217, incorporated by reference herein, and is generically known as a xe2x80x9cMoineauxe2x80x9d pump or motor. The mechanism has two helical gear members, where typically an inner gear member rotates within a stationary outer gear member. In some mechanisms, the outer gear member rotates while the inner gear member is stationary and in other mechanisms, the gear members counter rotate relative to each other. Typically, the outer gear member has one helical thread more than the inner gear member. The gear mechanism can operate as a pump for pumping fluids or as a motor through which fluids flow to rotate an inner gear so that torsional forces are produced on an output shaft. Therefore, the terms xe2x80x9cpumpxe2x80x9d and xe2x80x9cmotorxe2x80x9d will be used interchangeably herein.
FIG. 1 is a schematic cross sectional view of a pumping/power section of a PCP. FIG. 2 is a schematic cross sectional view of the PCP shown in FIG. 1. Similar elements are similarly numbered and the figures will be described in conjunction with each other. The pumping section 1 includes an outer stator 2 formed about an inner rotor 4. The stator 2 typically includes an outer shell 2a and an elastomeric member 10 formed therein. The rotor 4 includes a plurality of gear teeth 6 formed in a helical thread pattern around the circumference of the rotor. The stator 2 includes a plurality of gear teeth 8 for receiving the rotor gear teeth 6 and typically includes one more tooth for the stator than the number of gear teeth in the rotor. The rotor gear teeth 6 are produced with matching profiles and a similar helical thread pitch compared to the stator gear teeth 8 in the stator. Thus, the rotor 4 can be matched to and inserted within the stator 2. The rotor typically can have from one to nine teeth, although other numbers of teeth can be made.
Each rotor tooth forms a cavity with a corresponding portion of the stator tooth as the rotor rotates. The number of cavities, also known as stages, determines the amount of pressure that can be produced by the PCP. Typically, reduced or no clearance is allowed between the stator and rotor to reduce leakage and loss in pump efficiency and therefore the stator 2 typically includes the elastomeric member 10 in which the helical gear teeth 8 are formed. Alternatively, the elastomeric member 10 can be coupled to the rotor 4 and engage teeth formed on the stator 2 in similar fashion. The rotor 4 flexibly engages the elastomeric member 10 as the rotor turns within the stator 2 to effect a seal therebetween. The amount of flexible engagement is referred to as a compressive or interference fit.
FIG. 3 is a cross sectional schematic view of diameters of the stator shown in FIGS. 1 and 2. A typical stator 2 has a constant minor diameter 3a defined by a circle circumscribing an inner periphery of the stator teeth 8. The typical stator also has a constant major diameter 5a defined by a circle circumscribing an outer periphery of the teeth 8. A thread height 7a is the height of the teeth, which is the difference between the major diameter and the minor diameter divided by two, i.e., a minor radius subtracted from a major radius.
FIG. 4 is a cross sectional schematic view of diameters of the rotor shown in FIGS. 1 and 2. The rotor 4 has minor and major diameters and a thread height to correspond with the stator. The typical rotor has a minor diameter 3b defined by a circle circumscribing an inner periphery of the teeth 6. The rotor also has a major diameter 5b defined by a circle circumscribing an outer periphery of the teeth 6. The thread height 7b is the difference between the major diameter and the minor diameter divided by two.
A PCP used as a pump typically includes an input shaft 18 that is rotated at a remote location, such as a surface of a wellbore (not shown). The input shaft 18 is coupled to the rotor 4 and causes the rotor 4 to rotate within the stator 2, as well as precess around the circumference of the stator. Thus, at least one progressive cavity 16 is created that progresses along the length of the stator as the rotor is rotated therein. Fluid contained in the wellbore enters a first opening 12, progresses through the cavities, out a second opening 14 and is pumped through a conduit coupled to the PCP. Similarly, a PCP used as a motor allows fluid to flow from typically a tubing coupled to the PCP, such as coiled tubing, through the second opening 14, and into the PCP to create hydraulic pressure. The progressive cavity 16 created by the rotation moves the fluid toward the first opening 12 and is exhausted therethrough. The hydraulic pressure, causing the rotor 4 to rotate within the stator 2, provides output torque to an output shaft 19 used to rotate various tools attached to the motor.
The rubbing of the rotor in the stator as the rotor rotates causes several problems. Various operating conditions change the interference fit and therefore a predetermined amount of interference is difficult to obtain for efficient performance under the varying conditions. One problem is that a PCP can encounter fluctuations in operating temperatures. For example, some wellbore operations inject steam downhole through the pump into a production zone and then reverse the flow to pump production fluids produced by the wellbore at a different temperature up the wellbore. The temperature fluctuations can cause the components, particularly the elastomeric member, to expand and change the interference fit between the stator and rotor. Accordingly, because PCPs operate effectively only within a narrow range of fit, PCPs are limited to operations in which the temperature remains substantially constant.
One attempt to overcome the problems associated with the operation of a PCP in a variable temperature environment is to periodically change the pump components to accommodate the current ambient temperature. For example, the rotor may be periodically exchanged for a rotor with different dimensions in order to maintain the desired interference fit. The effectiveness of this practice is limited because a single rotor size can only accommodate a narrow temperature range, e.g., about 20xc2x0 C. to about 30xc2x0 C. As a result, the rod string must be pulled from the well bore and the rotors must be changed too frequently to be a practical solution.
Therefore, there exists a need for providing a PCP that can be adjusted to a variety of selected interference fits or even clearances to meet various operating conditions.
The present invention provides a self-compensating rotor and/or stator, so that the interference fit and/or clearance is maintained over a range of temperatures. The rotor and/or stator are tapered to provide a difference in fit between the rotor and stator by longitudinal adjustment of their relative position. In one embodiment, the adjustment may occur in response to a change in the length of a rod string while the PCP in mounted downhole in a wellbore.
In one aspect, a progressive cavity pump (PCP) having a inlet and an outlet is provided. The PCP comprises a stator defining a bore and a rotor slidably disposed in the bore. A shaft is connected to the rotor and has a length changing with temperature. The bore and the rotor are tapered at least partially between the inlet and the outlet so that a predetermined interference fit between the stator and the rotor is maintained during the change in the length.
In another aspect, a self-compensating progressive cavity pump (PCP) having a inlet and an outlet is provided. The PCP comprises a stator carrying an elastomeric member on an inner surface, wherein the elastomeric member has a thickness and a thermal expansion coefficient and wherein a surface of the elastomeric member defines a bore having an increasing diameter along at least a portion of its length. A rotor slidably disposed in the bore has at least a portion that increases diametrically along its length and has an outer surface defining a taper angle xcex8. The taper angle xcex8 is selected to maintain a predetermined interference fit between the stator and the rotor during relative axial movement therebetween. A shaft connected to the rotor has a length that increases with an increasing temperature, whereby the. rotor is axially moved relative to the stator when the stator is fixed in position. In one embodiment, the taper angle xcex8 is determined according to at least the thermal expansion coefficient of the elastomeric member, the thickness of the elastomeric member, the length of the shaft and a thermal expansion coefficient of the shaft.
In yet another aspect, a self-compensating progressive cavity pump (PCP) comprises a rotor disposed in the bore of a stator; wherein the stator and the rotor define interfacing inclining surfaces adapted to move over one another and defining an interference fit that is maintained while the rotor is axially reciprocating within the bore in response to a change in an ambient temperature.
In another aspect, a method of adjusting a progressive cavity pump as a function of temperature is provided. The method comprises a) providing a rotor slidably disposed in an opening of a stator, wherein the stator and rotor comprise interfacing inclined surfaces; b) axially moving the rotor and the stator relative to one another as a function of temperature; and c) maintaining a desired interference fit between the interfacing inclined surfaces while performing step b).